There are approximately 336 refrigerants used in a myriad of applications worldwide. Most are variations of the CNFMHIClJ molecule with a small number having trailing Br and O atoms attached. Of these, the most common and widely used are hydrofluorocarbons (HFCs) as represented by HFC-134A, HFC-152A, HFC-23, HFC-32, HFC-143A, HFC-125, HFC-245FA, HFC-227EA as their contribution to atmospheric ozone depletion is much smaller than most others.
Common HFCs such as HFC 134A, 407C, 410A and 152A are finding an increasing variety of applications related to comfort cooling chillers (residential, industrial or automotive), commercial refrigeration, and industrial process refrigeration where such refrigerants are required. This increased adoption is due to the fact that the more commonly used freons until recently contain chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs) which are being phased out in the United States and internationally due to their adverse effect (ozone depletion) on the earth's atmospheric environment. Therefore, HFCs have become much more prevalent in the applications mentioned above. Under section 608 of the Clean Air Act, it has been illegal since Nov. 15, 1995, to knowingly vent substitutes for CFC and HCFC refrigerants during the maintenance, service, repair and disposal of air-conditioning and refrigeration equipment. Therefore, though the HFCs are allowed as refrigerants, they cannot be vented to atmosphere either so their careful treatment is important.
The moisture content of these refrigerants is often critical to their application in such chiller applications as levels of frost and/or ice can form in expander valves and chambers should the moisture content be too high. Such freezing can seriously degrade or even destroy the process or the product in which they are employed and cause problems for maintenance or perhaps even unwanted venting prohibited by the above statutes. As a result, both producers and users have a strong need to determine the amount of moisture in the HFC they use or sell. Gas producers need to understand the trace moisture content of their gases so that they can assure themselves that products contain only the miniscule amounts of moisture they claim. Users need to measure moisture in these same gases to assure themselves that the stock gases being used do not adversely affect the product into which they are introduced.
Conventional techniques for measuring moisture in HFCs have significant disadvantages such as maintenance and calibration issues in the field that can be costly and time consuming to address.
Some conventional sensors for detecting moisture in refrigerants employ chilled sensors. With such devices, when gas flows over a chilled surface (such as a mirror) the moisture will condense on it—the exact temperature at which this condensation begins is the dew point. The temperature of the mirror is reduced from high to low (so that it passes through the dew point temperature), and the temperature is read exactly when the dew is observed. By obtaining the dew point temperature, one can calculate the moisture content in the gas. The mirror temperature is controlled by the flow of a refrigerant over the mirror or by using a thermoelectric cooler. The detection of condensation on the mirror can be done by eye or by optical means. For example, a light source can be reflected off the mirror into a detector and condensation detected by changes in light reflected from the mirror. The observation can also be done by eye; however the exact point at which condensation begins is not visible to the eye. Also, because the temperature is passing through the dew point rather than stopping exactly at the dew point, the measurement tends to be high and will have a high standard deviation. Additionally, the condensation of moisture can be confused with condensation of other condensable gases such as heavy hydrocarbons, alcohol, glycol or the refrigerant itself. Automated on-line systems are not able to make these distinctions and manual systems must be used only by highly skilled operators.
Other conventional sensors for detecting moisture in natural gas utilize an electrolytic sensor that includes two closely spaced, parallel windings coated with a thin film of Phosphorous Pentoxide (P2O5). As this coating absorbs incoming water vapor, an electrical potential is applied to the windings that electrolyzes the water to hydrogen and oxygen. The current consumed by the electrolysis determines the mass of water vapor entering the sensor. The flow rate and pressure of the incoming sample must be controlled precisely to maintain a standard sample mass flow rate into the sensor. With such a sensor, contamination from oils, liquids, glycols or the refrigerant itself on the windings will cause drift in the readings and damage to the sensor. The sensor cannot react to sudden changes in moisture, i.e. the reaction on the windings' surfaces takes some time to equalize. Large amounts of or large upward changes to the amount of water in the sample gas (called slugs) will wet the surface and can require tens of minutes or hours to “dry-down”.
Instruments utilizing a piezoelectric adsorption sensor that operate by comparing changes in frequency of hydroscopically coated quartz oscillators can also be used to detect moisture in refrigerants. As the mass of the crystal changes due to adsorption of water vapor, the resonant frequency of the quartz crystal changes. The sensor is a relative measurement so an integrated calibration system with desiccant dryers, permeation tubes and sample line switching is used to correlate the system on a frequent basis. Interference from glycol, methanol, and damage from hydrogen sulfide or other sulfides or other contaminants result in readings that cannot be relied on. Such a sensor requires a calibration system which is often imprecise and adds to the cost and mechanical complexity of a system. In addition, the labor for frequent replacement of desiccant dryers, permeation components, and the sensor heads greatly increase the operational costs. Moreover, slugs of water render the system nonfunctional for long periods of time as the sensor head has to “dry-down”.
An oxide sensor that is made up of an inert substrate material and two conductive layers. The sandwiched inert material is sensitive to humidity and may be used to detect moisture in refrigerants as its dielectric properties change with increased absorbance of moisture. With such a sensor, moisture molecules pass thru the pores on the surface layer and cause a change to a physical property of the layer beneath it. An example is an aluminum oxide sensor which has two metal layers forming the electrodes of a capacitor. The number of water molecules adsorbed will cause a change in the dielectric constant of the sensor. The sensor capacitance is correlated to the water concentration. Another similar sensor concept is a silicon oxide sensor. It is an optical device that changes it refractive index as water is absorbed into the sensitive layer. When light is reflected through the substrate, a wavelength shift can be detected on the output which can be precisely correlated to the moisture concentration. Fiber optic connector may be used to separate the sensor bead and the electronics.
With such a sensor, water molecules take time to enter and exit the pores so some wet-up and dry down delays will be observed, especially after a slug. Contaminants and corrosives may damage and clog the pores causing a “drift” in the calibration, but the sensor heads can be refurbished or replaced and will perform better in very clean gas streams. As with the piezoelectric and electrolytic sensors, the sensor is susceptible to interference from glycol and methanol, the calibration will drift as the sensor's surface becomes inactive due to damage or blockage, so the calibration is reliable only at the beginning of the sensor's life.
Optical sensors have also been developed for use in measuring gases within HFCs. Some implementations measure gases in refrigerants by determining the broadband spectrum of the refrigerant in the absence of a sensing reagent using a broadband detector and a monochrometer or some such selectable monochromatic light source and comparing the spectrum into that of various types of refrigerants. The refrigerant spectrum that matches the measured spectrum is the refrigerant present. With such a sensor, concentration of the refrigerant or the included trace gases can be determined, however, the lower detection limit of this type of device is greater than is needed in many applications.
Other optical sensors measure a continuous spectrum using broadband light that is directed to a series of detectors sensitive to a variety of pre-selected infrared light frequencies. Such frequencies are selected such that when a certain combination of relative absorption is seen, the refrigerant or included trace gas can be identified. Such an arrangement also has a higher detection limit than is desired for many applications. Moreover, only small segments of the broadband spectrum can be used for measurement, thereby leading to false readings and considerable measurement complexity if there is more than one gas type present, particularly when the absorption bands fall in the same place (i.e.—interferences).